A conventional electromechanical system, as utilized for example in the aerospace industry, includes a synchronous machine that generates multi-phase AC power from a rotating shaft, e.g., coupled to a gas turbine engine. In addition to operating in a generator mode, the synchronous machine operates as a starter (motor) to start the aircraft engine. Following a successful engine start, the system initiates the generator mode to supply multi-phase AC power to the aircraft power distribution system. When a synchronous starter-generator is used to start the engine, electrical power from a power source is coupled to the synchronous starter-generator via a main inverter, which supplies regulated AC power to the main stator coil of the machine. At the same time, an exciter inverter supplies single-phase AC power (e.g., at a constant frequency of 400 Hz) to a stator excitation winding of the machine. This produces, through the transformer effect, an electromagnetic field in rotor excitation windings at zero rotational speed. This field induces an AC voltage that is rectified by a rectifier on the rotor to establish a magnetic field in a main rotor winding. The interaction between the flux produced by the magnetic field in the main rotor winding and the stator currents (flux) established in the main stator coil produces torque for starting the engine. The main inverter varies the balanced multi-phase AC power (amplitude, frequency, and phase) output to the main stator coil as a function of rotor speed and position to control torque for the starter-generator.
Using electric starter-generators for aircraft engine start in this manner can save weight and lower operating cost as part of new system architectures. In some recent applications where AC electrical machines are used as starters for an aircraft engine (main or auxiliary power unit), a high starting torque is applied to the gearbox and indirectly to the engine. The impact torque created by electric starters is typically greater than that created by air turbine starters. In some cases, this high starting torque is the sum of the torque produced by multiple, e.g., two, starters. Because of the large torque applied, and the fact that multiple starters are sometimes used on the same gearbox during the engine start, the impact torque on the gearbox will adversely affect gear life.